Absence of Outbreeding Depression among Captive Chimpanzee (Pantroglodytes) Offspring of Different Subspecies

Matings between close relatives often result in inbreeding depression, typically characterized by elevated rates of infant mortality. But the importance of outbreeding depression (deleterious matings between genetically divergent animals) is poorly understood. Some researchers speculated that infant mortality among captive-born chimpanzees was elevated by outbreeding depression due to between-subspecies matings, while others denied the presence of an outbreeding effect. Until now, these assertions have had no empirical support either way. We used mtDNA sequencing and phylogenetic comparison to African chimpanzee mtDNA sequences of known provenance to infer the subspecies identities of African-born founders at 5 captive chimpanzee colonies. We then searched colony birth records to identify within- and between-subspecies matings and compare infant mortality rates. We identified 1402 maternity events with known outcome from parents of known subspecies, including 1325 within-subspecies and 77 between-subspecies maternities. Log-linear analysis of contingency table data revealed no significant difference in infant mortality from within-subspecies versus between-subspecies matings (χ2 = 2.059, P = 0.151; odds ratio α = 0.679). Mean infant survival was 67.9%. Interestingly, the trend in infant mortality was opposite to that predicted by an outbreeding hypothesis, since same-subspecies matings had higher overall mortality (33%) than did the presumably problematic between-subspecies matings (25%). Further analysis of infant survival data is needed to resolve this paradox.